The community size thing is going to be interesting as the space grows. The fact that there are functionally infinite name spaces means that "politics" doesn't just get to become the default politics discussion space for everyone wandering into the place. Lemmy.ca/c/politics can be a very different place than Lemmy.ml/c/politics, which will be very different from lemmy.world/c/politics, which will be very different again from beehaw.org/c/politics.
And you can suppose that everyone will just use the biggest one by default, but I don't think that's necessarily true. The biggest subreddit got that way predominantly because of their name, and there's a good chance that people'll see their local one first, not the biggest. Or that they'll see multiple of them, and end up engaging with multiple communities before they realize what's going on and settle on the one that suits them best.
There will always be a biggest, but there can be a larger number of smaller, lively communities because they don't need to take on names like "r/truepolitics" or "r/onguardforthee" (which is a so very discoverable and intuitive r/Canada alternative).
We'll have to see how the dynamics play out over time.